1. Field of the Invention
The present invention relates to a pigment dispersion composition for coloring porous anodic oxide films on aluminum by pigment deposition in the pores, which is adapted for bright and fast coloration of surfaces of aluminum or aluminum alloy materials for use as facing materials of buildings and the like. More particularly, the invention relates to a pigment dispersion composition for depositing pores existing in anodized oxide films on a surface of aluminum or aluminum alloy material by way of electrophoresis for coloration of the aluminum oxide film.
2. Related Art
Aluminum and alloys thereof are often used as facing materials for buildings. These metals are typically covered with aluminum oxide and, therefore, exhibit a reduced adhesion with a paint coating formed by an ordinary coating method. To overcome such a drawback, various coating methods have been proposed. For example, Japanese Patent No. 65742 discloses a method for coloring an aluminum material by immersing it in a solution containing a water-soluble or oil-soluble dye dissolved therein. Further, Japanese Examined Patent Publications No. 38-1715 (1963) , No. 44-12566 (1969) and No. 46-16566 (1971) disclose a method for inorganically coloring an aluminum material in an aqueous solution containing a metallic salt. Still further, Japanese Examined Patent Publications No. 51-35177 (1976) and No. 52-5010 (1977) disclose a method for allowing a pigment dispersed in water with a surface-active agent to be adsorbed by aluminum oxide formed on a surface of an aluminum material by way of electrophoresis. Yet further, Japanese Unexamined Patent Publication No. 5-93296 (1993) discloses a method for depositing pores existing in anodic oxide films formed on a surface of aluminum or aluminum alloy material with an aqueous dispersion of an organic pigment or carbon black by way of electrophoresis. In accordance with this method, the pores in the anodic oxide films are enlarged and then deposited with the aqueous dispersion of the organic pigment or carbon black. The aqueous dispersion is prepared by dispersing in water containing an anionic surface-active agent and carbon black or organic pigment which is finely disaggregated by first dissolving the organic pigment in concentrated sulfuric acid and then pouring it in water for precipitation or by way of a plasma treatment.
In accordance with the method disclosed in Japanese Patent No. 65742, however, the sunlight-fastness is unsatisfactory due to the use of a dye, though the aluminum material can be brightly colored.
In accordance with the method disclosed in Japanese Examined Patent Publication No. 38-1715, the light-fastness is excellent by virtue of the use of an inorganic compound. However, the method suffers such inconveniences as limited color variation and insufficient brightness in coloring an aluminum material.
Where an aluminum material is anodized by an ordinary method, pores existing in the resultant aluminum oxide film typically have diameters of not greater than 50 nm when formed by way of a low-voltage anodization, or have diameters of about 180 nm when formed by way of a high-voltage anodization. In accordance with the method disclosed in Japanese Examined Patent Publication No. 51-35177, the pores in the anodic oxide film formed on the aluminum material by an ordinary method are deposited by way of electrophoresis with a pigment dispersion in which pigment particles having particle diameters of not greater than 1,000 nm are dispersed in water using a surface-active agent. Thus, the pigment particles used are large in diameter relative to the pores in the anodic oxide film, posing the following problems. The pigment particles, though having a wide particle-size distribution, mostly have sizes larger than the pores. Therefore, a very small number of pigment particles can migrate into the pores by way of electrophoresis, while the major part of pigment particles having larger sizes are adsorbed and stay around the mouths of the pores. This prevents micro-sized pigment particles from migrating into the pores, and causes the pigment coat to be easily scaled off during a washing process performed after the coloring process, resulting in a coloring failure. Further, the dispersant used to disperse the pigment remains in the pores. This may deteriorate the resistance of the aluminum oxide film to solvents and other chemicals used for the coloration.
In accordance with the method disclosed in Japanese Unexamined Patent Publication No. 5-93296, the particle size of the organic pigment is reduced to a level smaller than the diameters of the pores by way of the plasma treatment or by first dissolving the organic pigment in a 98% conc. sulfuric acid and then pouring it in water for microparticulate precipitation, before the pigment is dispersed in water. Though the pigment finely disaggregated by way of the aforesaid pretreatments can deeply migrate into the pores of the aluminum oxide film, the preparation of the dispersion is not necessarily satisfactory. In addition, these pretreatments may cause hazards resulting from the use of plasma wave or concentrated sulfuric acid, thereby posing an operational problem. Furthermore, these pretreatments accompany a side reaction by which the pigment is rendered into a substance susceptible to ionization. This may cause contamination by ionic inclusions which deteriorate the stability of the dispersion.
As can be understood from the foregoing, it is required to reduce the diameter of pigment particles dispersed in a dispersion to a level smaller than that of pores existing in anodic oxide films, in order for the pigment particles to migrate into the pores to a satisfactory depth. However, the preparation of the dispersion satisfying such requirements is not easy.
It is, therefore, an object of the present invention to provide a pigment dispersion composition which is suitable for depositing in pores existing in anodic oxide films for coloration and has an excellent property to meet requirements for repeated coloration.
In general, pigments each have a characteristic particle-size distribution. Where a commercially available pigment is used without being subjected to a preliminary fine-disaggregation process, a fraction of pigment particles having particle sizes smaller than those of pores in anodic oxide films may enter the pores to some extent. However, a major fraction of pigment particles having larger particle sizes may cause problems as previously mentioned. Further, pigments having the same average particle size may have different particle-size distributions. This means that the content of pigment particles having a larger size varies depending on the particle-size distribution of the pigment and, therefore, the average particle size does not necessarily properly represent the particle-size distribution.
As a result of intensive study, the present inventors have found a particle size and other physical characteristics of a pigment dispersion composition suitable for depositing in pores in anodic oxide films for coloration and a method for preparing such a pigment dispersion composition.